This is to request funds to purchase a molecular graphics system for Biomedical Research on the Berkeley campus of the University of California. Structural studies of biological molecules such as DNA, RNA and proteins cannot be done adequately without an interactive molecular graphics system. There are four areas of research that most require such a system. (1) Determination of three-dimensional structures of proteins and nucleic acids: Interpretation of electron density map from X-ray crystallographic data; Interactive refinement of molecular models to improve the fitting to the experimental data. (2) Molecular model building: Construction and display of molecular models such as secondary structure, loop structure, tertiary structure of RNA and DNA based on stereochemical, thermodynamic, biochemical and sequence complementarity criteria; display and examination of 3-D structures whose atomic coordinates are available; examination of possible structural changes due to sequence changes in mutant proteins. (3) Molecular dynamics: To enhance understanding of dynamic motion of biological models, it is necessary to produce moving pictures that are compatible with the experimental spectrographic and anisotropic thermal parameters.